1. Field of the Invention
The invention relates to compositions and to a process for generating colorless sealed layers on anodized aluminum surfaces in the course of "cold sealing".
2. Statement of the Related Art
In contrast to "hot sealing", in which the pores of anodized aluminum surfaces are closed and rendered corrosion resistant by treatment with water, steam, or metal salt solutions at a temperature above 90.degree. C., in the art the terms "cold sealing" or "cold impregnation" or "low temperature sealing" are generally understood to denote processes by which the porosity of anodized aluminum surfaces is reduced at a temperature of 15.degree. C. to 70.degree. C., and the surface properties thereof are substantially improved. These processes are intended to provide improved corrosion resistance over that of unsealed surfaces.
The underlying mechanisms of the actual sealing process have so far not been elucidated in all details. Nevertheless, it can be said that in the region adjacent to the surface of the aluminum oxide layer which is formed on aluminum metal upon contact with oxygen the pores are closed by the incorporation of aluminum oxide hydrates, e.g. boehmite. However, in the course of sealing it is undesirable that such a mineral coating is formed on the surface of the oxide layer as well, as this coating is not resistant to handling and the surfaces of the anodized aluminum parts will become spotty and defective in appearance. Thus, methods for sealing metallic surfaces by forming additional inorganic protective layers such as described in U.S. Pat. No. 3,012,917 have not been accepted in technical applications.
Processes to effect cold sealing of anodized work pieces made of aluminum and alloys thereof are known in the prior art. Thus, Chemical Abstracts, 87, 75493t (1977) describes employing solutions of various metal fluorides, for example CrF.sub.3, MnF.sub.2, CoF.sub.2 or NiF.sub.2, for treating anodized aluminum surfaces at from room temperature to 50.degree. C. Published Japanese patent application No. 50-117,648 describes the sealing of anodized aluminum surfaces by immersion into an acid solution containing a metal (such as nickel) fluoride and isoamyl alcohol at 30.degree. C.
Published British patent application No. 2,137,657 (and corresponding German patent application No. 33 01 507) also describe a process for cold sealing of aluminum or aluminum alloy surfaces in which process solutions containing fluorine or fluoride or complex fluoride anions (such as nickel fluoride and/or cobalt fluoride) are employed at temperatures of 25.degree. to 60.degree. C. Similarly, published U.K. patent application No. 2,140,033 (and corresponding published German patent application No. 34 11 678) disclose a process for after-sealing of aluminum and aluminum alloys subsequent to electrolytic anodization wherein nonionic surfactants capable of reducing the surface tension of the sealant bath are added to aqueous solutions containing at least one nickel salt. Useful nonionic surfactants include fluorocarboxylates and organosiloxanes.
All of the described processes have in common that aqueous solutions of certain nickel salts are used. The nickel ions are incorporated in the surface layer upon contact with the freshly anodized aluminum surfaces. In consequence thereof, depending on the types of ions also included, a more or less intense greenish coloration of the aluminum surfaces occurs, which is particularly cleanly visible from either an oblique top view or at an acute view angle. In work pieces for decorative use, the greenish surface discoloration is very annoying, since thereby the actual natural shade of the aluminum metal is changed.
Processes for removing or eliminating undesired colorations or discolorations of anodized surfaces of certain aluminum alloys have been described in U.S. Pat. No. 3,874,902 (and corresponding published German patent application No. 25 10 246). In the process disclosed, alterations of the color of aluminum surfaces resulting from alien metals of the aluminum alloys, e.g. copper, are eliminated by adding a monoazo dye to the bath at a temperature which must be maintained at about 180.degree.-210.degree. F. (82.degree.-98.degree. C.) and at a pH which must be between about 5.5 and 6.5, for a time of 10-30 minutes. However, the disclosure teaches removing only those discolorations which have been formed from nonaluminum components of the aluminum alloy in the anodization step preceding the sealing. As treatment accelerators there can be added metal salts, such as cobalt or nickel salts. However, the addition of accelerators renders the color control difficult.
In the aluminum industry, monoazo dyes are mainly used to provide anodized aluminum surfaces with a desired color. The dyes penetrate into and are adsorbed in the porous surface layers of anodized aluminum parts, after which the colored layers primarily are sealed by treatment with hot aqueous solutions. The sealing solutions are at the same time provided with further additives which will hinder the formation of sealing coatings [see "Aluminium" 47, 245 (1971)]. In those cases, the addition of low amounts of nickel salts, such as nickel acetate, or pre-treatment of the surfaces with nickel salt-containing solutions are often required to prevent the dyes from exudation from the pores and to avoid an undesirable alteration in the color shades as imparted by the organic dyes.
However, in cold sealing using solutions containing nickel salts, an undesired greenish discoloration of the surface occurs due to the components of the solutions required for the sealing process.